Cortical topography: sulci and gyri on MRI
On axial and sagittal MRI slices (T1 and T2-weighted), key cortical elements can be clearly differentiated. The superior frontal sulcus (sulcus frontalis superior) is visualized between the superior and middle frontal gyri. It merges almost at a right angle with the precentral sulcus (sulcus precentralis). Posterior to the latter lies the central sulcus (sulcus centralis). Within the precentral gyrus itself (gyrus precentralis), a characteristic thickening (protrusion) can be found—this is the motor hand area.
On the medial surface along the corpus callosum runs the cingulate sulcus (sulcus cinguli). Approaching the splenium of the corpus callosum (splenium), it turns toward the brain surface, forming the marginal branch (pars marginalis sulci cinguli), which lies directly posterior to the central sulcus.
In the parietal lobe, the landmark is the intraparietal sulcus (sulcus intraparietalis), which separates the superior (medial) and inferior (lateral) parietal lobules. Anteriorly, it merges with the postcentral sulcus (sulcus postcentralis).
The basal parts of the frontal lobe are notable for orbital sulci (sulci orbitales) forming a characteristic H-shape. They divide the orbital gyri (gyri orbitales) into anterior, posterior, medial, and lateral parts. The most medial structure on the basal surface is the straight gyrus (gyrus rectus), separated from the medial orbital gyrus by the olfactory sulcus (sulcus olfactorius). The frontal and temporal lobes are clearly separated by the Sylvian fissure (lateral sulcus).
Basal ganglia, capsules, and white matter
Deep brain structures are clearly visible on axial CT and T2-weighted MRI. The internal capsule is V-shaped and sandwiched between the thalamus (thalamus), caudate nucleus (nucleus caudatus), and lentiform nucleus (nucleus lentiformis). On parasagittal slices, the basal ganglia appear "striated" due to bridges of gray matter that cross the internal capsule and connect the caudate and lentiform nuclei.
Special attention is paid to the claustrum (claustrum), a very thin sheet of gray matter lying between the insular cortex (insula) and the lentiform nucleus. It strictly separates the white matter of the external and extreme capsules.
The white matter forms the semioval center (centrum semiovale), which contains several types of tracts: association, commissural, ascending/descending, and subcortical U-shaped fibers. On coronal sections, one can distinguish bundles of the cingulum (cingulum) passing within the cingulate gyrus, and the superior longitudinal fasciculus.
Midline structures, hypothalamus, and ventricular system
On the sagittal MRI slice, parts of the corpus callosum (corpus callosum) are visualized: rostrum, genu, body, and splenium. Nearby, the anterior and posterior commissures (the latter forming the posterior boundary of the cerebral aqueduct) can be traced, as well as the fornix (fornix) and the interthalamic adhesion (massa intermedia) within the third ventricle.
The hypothalamus (hypothalamus) forms the floor and anterior parts of the lateral walls of the third ventricle, extending obliquely downward from the foramen of Monro to the cerebral aqueduct. It is a group of nuclei responsible for the regulation of autonomic functions, appetite, and hormone production. It is closely connected to the limbic system and serves as its effector "output". A hollow infundibulum extends inferiorly from the hypothalamus, leading to the pituitary gland (pituitary gland), which is located in the hypophyseal fossa and may vary in size.
Limbic system and age-related changes on CT
Components of the limbic system, which is responsible for species preservation, are located in the medial parts of the temporal lobe. It is a misconception to consider it exclusively an ancient structure: for example, the hippocampus (hippocampus) is an evolutionary innovation of mammals. Coronal MRI slices clearly show its head, body, and tail, as well as the parahippocampal gyrus.
The most anterior structure of the complex is the amygdala (amygdala), separated from the head of the hippocampus by the uncal recess of the temporal horn. Located medially is the uncus (uncus), consisting of amygdaloid and hippocampal components. The connection of the hippocampus with other areas is provided by the fimbria-fornix (fimbria-fornix) system.
When analyzing CT scans, it is important to consider typical physiological calcifications that normally occur with age:
- Globus pallidus (globus pallidus) — bilateral calcification.
- Pineal gland (pineal gland) — midline calcification (paired internal cerebral veins lie anterior to it).
- Choroid plexuses (choroid plexuses) — calcification in the region of the trigones of the lateral ventricles.
Briefly: answers to topical questions
Which nuclei are part of the hypothalamus?
The hypothalamus includes nuclei of the anterior, middle, and posterior groups.
- Preoptic nucleus (Preoptic nuclei) — belongs to the anterior group.
- Paraventricular nucleus (paraventricular) — belongs to the anterior group.
- Supraoptic nucleus (supraoptic) — belongs to the anterior group.
- Dorsomedial nucleus (Dorsomedial) — belongs to the middle (tuberal) group.
- Ventromedial nucleus (ventromedial) — belongs to the middle group.
- Infundibular nucleus / arcuate nucleus (arcuate) — belongs to the middle group.
- Tuberomammillary nuclei — belong to the middle group.
- Posterior hypothalamic nucleus (Posterior hypothalamic nucleus) — belongs to the posterior group.
The posterior group also includes the mamillary body (Mammillary body).
Which anatomical structures form the walls of the third ventricle?
The walls and floor of the third ventricle are formed by structures of the diencephalon and the terminal lamina.
- Thalami (Thalami) — their massive gray matter forms the major part of the lateral walls.
- Hypothalamus (hypothalamus) — forms the floor of the third ventricle and its lateral walls anteriorly.
- Lamina terminalis (lamina terminalis) — forms the anterior wall of the third ventricle in adults.
The ventricular cavity also contains the interthalamic adhesion (massa intermedia), connecting the thalami.
What structures are part of the limbic system?
The limbic system includes cortical divisions and subcortical formations.
- Hypothalamus (Hypothalamus) — the central link of autonomic regulation.
- Septum (Septum) — including the septum pellucidum and septal nuclei.
- Amygdala (Corpus amygdaloideum / amygdala) — a subcortical structure.
- Hippocampus (Hippocampus) — belongs to the allocortex (archicortex).
- Cingulate gyrus (Gyrus cinguli) — belongs to the allocortex.
- Parahippocampal gyrus (parahippocampal gyrus) — belongs to the mesocortex.
The system also includes the olfactory bulbs and tubercle, dentate gyrus, insular cortex, anterior thalamic nucleus, and mamillary bodies.
Orbital gyri are easy to remember by the initial letters A, P, M, L (Anterior, Posterior, Medialis, Lateralis) — they are separated from each other by sulci forming an "H" shape.
Class and exam questions
Why might the interventricular foramen of Monro appear dark on T2-WI?
This is due to the flow void phenomenon caused by the directed flow of cerebrospinal fluid through the foramen.
Where on tomograms should the claustrum be looked for?
The claustrum is located lateral to the lentiform nucleus and medial to the insular cortex, dividing the white matter into the external and extreme capsules.
In which brain structures are physiological calcifications normally detected on CT?
Age-related physiological calcification is typical for the pineal gland (along the midline), globus pallidus, and choroid plexuses in the trigones of the lateral ventricles.
What else to study on this topic
- Radiation anatomy of the brainstem and cranial nerves
- Connections and pathways of the limbic system: Papez circuit
- Angiography: arterial territories and dural venous sinuses of the brain
- Imaging appearance of meningeal spaces and cisterns
- Imaging features of the indusium griseum on high-field MRI
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